Mass spectrometer



April 16, 1963 A. M. KLOPFER 3,086,110

MAss SPECTROMETER 2 sheets-sen 1 FIG2 f )Q 2 FIG. 3 /2 INVENTOR ANTON MARTIN KLOPFER April 16, 1963 Filed March 2. 1959 2 SheetSLShee'l'. 2

FIG. 6

INVENTOR` ANTON MARTI N KLOPFER BY f/,M a W AGENT United States Patent O 3,086,110 MASS SPECTROMETER Anton Martin Klopter, Aachen, Germany, assigner t North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Fitted Mar. 2, 1959, Ser. No. 796,626

Claims priority, application Germany Mar. 15, 1958 1 Claim. (Cl. Z50-41.9)

This invention relates to mass spectrometers for gases at low pressures in which a beam of electrons is shot in the direction of a static homogeneous magnetic field between two relatively parallel and electrically connected plates, an electric high-frequency iield being produced in the space between said plates, at right angles to the magnetic field, between two likewise parallel plates, annular intermediate electrodes being provided for homogenising the high-frequency field and adjacent one high-frequency electrode a collector being arranged for such ions as describe paths of increasing radius by the action of the various fields.

In one of the oldest embodiments of the above-mentioned mass spectrometer, which is also known under the name omegatron, the high-frequency plates are collectors. Between these plates there `also exists a weak static field which serves to deflect non-resonant ions at right angles to the direction of the magnetic field and of the electric high-frequency field. If desired the two high-frequency plates may be connected separately to negative voltages with respect to the field-distribution rings.

In another embodiment, one of the field distribution electrodes serves also as a collector for the resonant ions.

A construction is also known which has no -tield distribution rings, but in which the two plates between which the beam of electrons is accelerated, together with two plates at right angles thereto, embrace the high-frequency space. In certain cases, these four plates have applied to them a common negative voltage less than 1 volt with respect to the high-frequency plates (on account of sensitivity variations of the omegatron).

Furthermore, a construction is known which has either field distribution rings or side plates embracing the highfrequency field. The collector is constituted by a strip parallel to the high-frequency plates, the strip being either situated between the field distribution rings, or introduced into a slot in a side plate. rIhe electrodes ernbracing the high-frequency space are at a weakly positive potential with respect to the high-frequency plates.

It has also been described to provide the high-frequency plates with a U shape.

It has been found that various types of omegatrons do not yield reproducible results especially if the partial pressures of the components of a mixture of gases are to be determined.

An object of the invention is to provide an improvement which allows of reproducible measurements.

ln a mass spectrometer for gases of low pressure, in which a beam of electrons is shot in the direction of a statical homogeneous magnetic eld between two relatively parallel and electrically connected plates, in which an electric high-frequency field is produced in the space between these plates, at right angles to the magnetic field, between two likewise parallel plates, in which annular intermediate electrodes are provided for homogenising the high-frequency field and in which adjacent one high-frequency electrode a collector is arranged for such ions as describe paths of increasing radius by the action of the various fields, and in which the electrodes embracing the high-frequency space have applied -to them a weakly positive potential wtih respect to the 4high-fre- 3,086,l l0 Patented Apr. 16, 1963 ICC quency plates, according to the invention two relatively parallel plates are arranged parallel -to the direction of the magnetic field and at right angles to the field-distribution rings adjacent the ends of these rings, said plates having applied to them a negative voltage of several -tens of volts with respect to the high-frequency plates so that the tiow of ions towards the collector isa maximum for both light and heavy ions.

The present invention builds further upon the recognition that a positive electrostatic field between the high-frequency plates and the field-distribution rings, possibly the plates through which the beam of electrons enters and emerges, facilitates a movement of ions towards the collector in planes parallel to the direction of the magnetic field, whereas this field causes ions to move towards the other electrodes in planes at right angles to the magnetic field. The last-mentioned unfavorable iield is not only lavoided due to the two side-plates being at negative potential, but also the ions are assisted in their movement towards the collector, so that interfering effects, such as gas layers and electric charges of the electrodes, cannot substantially influence the paths of the ions.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

FIGS. l 4to 3 show three mutually perpendicular crosssections of a mass spectrometer according to the invention;

FIGS. 4 and 5 show its circuit diagrams and FIG. i6 shows the relative liow of ions towards the collector` as a function of a negative voltage on the side plates, that is to say for ions of two different masses.

In F-IGS. 1 to 3, the bulb of the -mass spectrometer is indicated by 1. A plurality of current-supply wires are `arranged in the base of bulb 1. The electrode system is built up on these lead-through wires with the aid of wire supports. The base has a tubular extension 2 in which the current-supply wire for the collector extends. The cathode filament, indicated by 3, is arranged inside a screen electrode 4 having an aperture for the beam of electrons. An accelerating and focusing electrode 5 is arranged in front of the aperture in screen 4. The two plates 6` provided with apertures for passage of the electron beam are connected to one high-frequency plate 7, the other high-frequency plate bearing the number 8i. The plate 7 has a narrow aperture 9 through which a collector 10 is passed. The current-supply wire 11 for collector 10 is surrounded by a screen structure 13.

The field-distribution rings, which are in total four in the figure, but may alternatively be six or eight in number, are indicated by 12. A collector 14 for the electrons is .arranged behind one of the plates 6. The side plates according to the invention bear the number 15.

A tube leads from the bulb to the apparatus from the contents of which the composition of the gas is to be determined.

The direction of the magnetic field produced by polepieces 33, 34 is indicated by arrow H in FIG. l. The figures show the correct relative proportions. Plate S has dimensions of 25 by l5 mms.

The electrodes consist of gold sheet of 0.3 mm. thick. lf desired, it is possible to use gold-plated constantan or platinum or a platinum alloy. If the mass spectrometer can be sufficiently protected against oxidation, constantan is also permissible as the electrode material.

In FlG. 4, reference numeral 21 indicates the heatingcurrent battery for -the cathode and 21 indicates the battery for the accelerating voltage. The battery 23 applies to the electrode 5 a positive voltage of about 10 volts with respect to the electrode 6. The electrodes 4 and 5, when suitably connected, may be used for stabilising the flow of electrons. The battery 22 has a voltage of about )y volts. The collector-electrode 14 receives, with the aid of `a battery 24 and a potentiometer 25, a positive voltage of at most about 40 volts with respect to the electrode 6.

In FIG. 5, the high-frequency voltage is applied to a terminal 26. The maximum effective high-frequency voltage is 1 volt and the frequency lies between 8 mc./Sec. and 30 kc./sec. for the masses of from 1 to 260. The magnetic eld has a strength of 5,000 gauss.

The tield-distribution rings 12 are coupled by means of resistors 27 each of 200` ohms and capacitors 23 each of 1 nf. in a high-frequency manner to the plates i and 8. By means of a battery 29 and a voltage divider 30, the rings 12 receive a positive voltage of at most 1 volt with respect to the plates 7 and 8.

A battery 31 and a potentiometer 32 impose on the plates 1S a negative voltage of at most 200 volts with respect to the electrodes 7 and 8.

FIG. 6 shows the relative flow of ions z'+/i+max towards the collector for the gases nitrogen and hydrogen as a function of the negative voltage V15 on the electrode 15 at a pressure of 4.1041 mm. of Hg with otherwise optimum conditions for a mass spectrometer having 6 field-distribution rings. It will readily be evident that a rather high negative voltage on the electrodes 15 is required for both the light and the heavy ions. The value of the voltage is, of course, dependent upon the distance between these electrodes and the field-distribution rings, upon the dimensions and the number of these rings. A beam of electrons is drawn from cathode 3 through holes in shield 4, electrode 5 and rst electrode 6 in the space between high-frequency electrodes 8, 9 and eld rings 12. The electron beam ionizes the gas present in enclosure 1. The beam of electrons is collected through a hole in second electrode 6 on a collector electrode 14.

The ions formed come under the influence of the highfrequency eld between electrodes 8 and 9.

Due to the presence of the magnetic field H, the ions obtain movements perpendicular to the magnetic eld H. For certain ions for which the motion in the field corresponds to the frequency of the high-frequency field the movement is in spirals of increasing radius such as is known in the arrangements of the known art. Ultimately these ions are collected on collector electrode v110. By varying the `frequency of the high-frequency eld, ions of different mass can be collected.

From FIG. 6 it can be seen that a rather high negative Voltage on electrodes 15 is necessary to collect both light and heavy ions which could not be the case in the known arrangements.

What is claimed is:

A mass spectrometer for a gas at low pressures comprising means including a source of electrons and an accelerating electrode to project a beam of electrons into a gas in a given direction between a pair of spaced, rcla tively parallel, electrically connected plates; means to produce a static, homogeneous magnetic `lield between the plates in the same direction as said given direction; means to produce a high-frequency electric field between a second pair of plates in a direction perpendicular to the direction of the magnetic iicld; a plurality of annular electrodes disposed between the latter plates for homogenizing the high-frequency electric field; a co1- lector electrode disposed adjacent to one of said plates for the high-frequency electric eld to collect ions whose .paths have an increasing radius when subject to said field; means to apply a positive potential to the accelerating electrode relative to the electrodes enclosing the space; a pair of relatively parallel plates disposed parallel to the direction of the magnetic field adjacent to the ends of and at right `angles to the annular electrodes; and means .to apply a relatively large negative potential to said latter plates to maximize the flow of light and heavy ions to the collector.

References Cited in the tile of this patent UNITED STATES PATENTS 2,629,055 Robinson Feb. 17, 1953 2,698,389 Jernakoff Dec. 28, 1954 2,798,956 Lanneau et al July 9, 1957 2,806,955 Langmuir Sept. 17, 1957 2,868,986 Lanneau et al. Jan. 13, 1959 

